net/rds: zero last byte for strncpy

for NUL terminated string, need be always sure '\0' in the end.

additional info:
  strncpy will pads with zeroes to the end of the given buffer.
  should initialise every bit of memory that is going to be copied to userland

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>